Aircraft engine nacelles provide streamlined enclosures for aircraft engines. The nacelles typically include an underlying support structure covered by a thin, aerodynamically shaped metal skin. The portion of the nacelle that surrounds an engine's inlet commonly is referred to as the nacelle inlet nose lip, or simply the noselip. The noselip has a complex shape with compound curvatures. First, the noselip has a chordwise curvature that curves from forward portions of the noselip toward aft portions of the noselip, thereby forming an aerodynamic shape. In addition, the noselip has a spanwise curvature that curves in a circumferential direction around the inlet. The noselip has a relatively large depth-to-diameter ratio. For example, the noselip may have a depth-to-diameter ratio of between about 1.0 and about 5.0. The compound curved shape of the noselip, the noselip's large depth-to-diameter ratio, and the large overall diameter of a noselip for high bypass ratio aircraft engines (up to 10 feet in diameter) can make the noselip particularly difficult to manufacture. Noselips commonly are produced in multiple arcuate segments to facilitate their manufacture and maintainability. The arcuate segments are assembled together in a conventional manner known to those skilled in the art to form a complete noselip.
Draw forming is one traditional method used to produce a sheet metal skin segment having a complex, multi-curved shape, and a large depth-to-diameter ratio. The draw forming process plastically deforms a sheet of metal by fixing the edges of the metal, and plunging a specially constructed die or punch into the sheet. The die has a shape corresponding to the desired shape of the formed metal. Optionally, the sheet of metal may be preheated before forming. The deep drawing process often requires multiple drawing cycles to produce a finally formed part. Unfortunately, the draw forming process is complex and time consuming. In addition, the draw dies used in the draw forming process experience substantial wear, and require periodic refurbishment or replacement. Furthermore, the tooling and equipment required to draw form a nacelle noselip, for example, can be expensive to purchase and costly to maintain.
Another common method of forming a complex skin segment having a large depth-to-diameter ratio is spin forming. Spin forming involves spinning a thin-walled workpiece on a rotating mandrel while heating and deforming the workpiece. Spin forming permits formation of a complete nacelle noselip in a single piece. The spin formed workpiece can be finally shaped during spin forming, or can be preformed by spin forming and finally shaped on a drop hammer die or the like. Unfortunately, the equipment and tooling required to spin form a part as large as a nacelle noselip can be expensive to purchase, and costly to maintain.
Thus, there is a need for an alternative, less costly, and less time-consuming process for producing metal skins having complex shapes and large depth-to-diameter ratios, such as nacelle inlet noselips.